AggregateListener (OpenJPA Parent POM 4.0.0 API)

All Superinterfaces:

java.io.Serializable

All Known Subinterfaces:

JDBCAggregateListener
```
public interface AggregateListener
extends java.io.Serializable
```
An aggregate listener aggregates data produced by an ExpressionQuery.

Author:

Abe White, Patrick Linskey

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`java.lang.Object`	`evaluate(java.util.Collection args, java.lang.Class[] argClasses, java.util.Collection candidates, StoreContext ctx)`	Return the value of this aggregate.
`boolean`	`expectsArguments()`	Return whether this function expects to act on arguments.
`java.lang.String`	`getTag()`	Return the tag that this listener listens for.
`java.lang.Class`	`getType(java.lang.Class[] argClasses)`	Return the expected type of the result of this listener.

- Method Detail
  - getTag
```
java.lang.String getTag()
```
    Return the tag that this listener listens for.
  - expectsArguments
```
boolean expectsArguments()
```
    Return whether this function expects to act on arguments. Some function may not need arguments; for example, a function to count the number of objects that match a given filter might be of the form: count().
  - evaluate
```
java.lang.Object evaluate(java.util.Collection args,
                          java.lang.Class[] argClasses,
                          java.util.Collection candidates,
                          StoreContext ctx)
```
    Return the value of this aggregate.
    
    Parameters:
    
    args - for each candidate, the value of the arguments to the function; will be null if this aggregate does not expect an argument; if this function has multiple arguments, each element will be an array
    
    argClasses - the expected class of each argument element
    
    candidates - the candidate objects being evaluated
    
    ctx - the persistence context
    
    Returns:
    
    the value of the aggregate
    
    Throws:
    
    UserException - if this aggregate does not support in-memory operation
  - getType
```
java.lang.Class getType(java.lang.Class[] argClasses)
```
    Return the expected type of the result of this listener.
    
    Parameters:
    
    argClasses - the expected classes of the argument, or null if no arguments